In recent years, as networks have operated at high speeds and with high capacities, the importance of coherent optical communication in optical transmission systems has increased. This means that coherent optical communication has excellent optical noise resistance, nonlinearity resistance, and excellent frequency use efficiency. In addition, as a receiving format for coherent optical communication, a digital coherent format that can correct dispersion distortion that occurs in the waveforms of optical signals and that uses a small number of optical parts is increasingly attractive.
In the digital coherent format, the receiver is provided with an A/D converter and a DSP (Digital Signal Processor). A received optical signal is sampled based on a sampling clock by the A/D converter so as to convert the received optical signal as an analog signal into a digital signal. Thereafter, the digital signal is processed by the DSP so at to demodulate the digital signal.
To secure the reliability of optical communication in the digital coherent format, it is important to perform A/D conversion such that information with respect to the intensity and phase of the received optical signal is not lost.
Generally, when the sampling rate of the A/D converter is caused to be sufficiently higher than the symbol rate of the received optical signal, the A/D conversion can be performed without loss of information of the received optical signal.
However, because of technical restrictions such as performance limits of various devices provided in the receiver, it is difficult to cause the sampling rate to be sufficiently higher than the symbol rate of the received optical signal. Even if the sampling rate were caused to be sufficiently high, since the load imposed on the DSP for the digital signal process increases, the area of the principal circuit section and the drive frequency of the DSP would rise, resulting in an increase of the circuit area of the DSP and a rise of the cost.
Because of technical restrictions and problems in which there is an increase in the size of the circuit area of the DSP and a cost increase, it is not realistic to make the sampling rate sufficiently higher than the symbol rate of the optical signal.
Thus, it is desired to lower the sampling rate without loss of information of the received optical signal. It is further desired to cause the sampling rate to be equal to the symbol rate of the received optical signal. The sampling performed at a sampling rate equal to the symbol rate of the received optical signal is referred to as baud rate sampling.
To perform A/D conversion without loss of information of the received optical signal based on the baud rate sampling, it is necessary to ensure that the received optical signal is phase-synchronized with the sampling clock. Unless it is made certain the received optical signal is phase-synchronized with the sampling click, since information of the received optical signal cannot be digitized with a high signal-to-noise ratio, the error rate of the received optical signal rises.
As a technique that secures that the received signal is phase-synchronized with the sampling clock, for example, a clock reproduction circuit as presented in Patent Literature 1 is known.
In the clock reproduction circuit presented in Patent Literature 1, an A/D converter samples an IF signal (corresponding to the received optical signal) based on a sampling clock so as to perform the A/D conversion for the IF signal. The IF signal that has been converted from an analog signal into a digital signal is demodulated by a digital signal process and is thereby converted into a base band signal. Thereafter, the clock reproduction circuit adjusts a timing signal of the IF signal based on the base band signal so as to phase-synchronize the IF signal with the sampling clock.
However, the clock reproduction circuit presented in Patent Literature 1 assumes that even if the timing signal of the IF signal varies after the IF signal has been phase-synchronized with the sampling clock, the IF signal will be phase-synchronized with the sampling clock. Thus, in the initial state where a signal is initially received, the base band signal is demodulated based on an inadequate sampling clock that has not been phase-synchronized with the sampling clock. Thus, in the initial state, even if the sampling clock is generated based on the base band signal, the IF signal will not be phase-synchronized with the sampling clock.
As a technique that allows the received optical signal to be phase-synchronized with the sampling clock even in the initial state, an optical receiving device presented in Patent Literature 2 is known.
FIG. 1 is a block diagram showing a structure of the optical receiving device presented in Patent Literature 2.
In the optical receiving device, high speed A/D conversion section 104a samples a received signal received by light reception section 103 in synchronization with a sampling clock so as to perform the A/D conversion for the received signal. Digital filter section 105 corrects the waveform in distortion of the received signal that has been converted into a digital signal so as to reproduce the received signal.
Control value computation section 108 extracts error information that represents an error between an optimum timing and the sampling timing from the signal that has been corrected with respect to the waveform distortion by digital filter section 105. Voltage control oscillation section 104b adjusts the phase of the sampling clock based on the error information so as to phase-synchronize the received signal with the sampling clock.
The above-described process of the optical receiving device assumes that the waveform distortion correction performed by digital filter section 105 is correct to some extent. Thus, in the initial state where the signal is initially received, since the tap coefficients of digital filter section 105 are inadequate values, the waveform distortion cannot be corrected and thereby error information that represents inadequate sampling timing is likely to be extracted.
Thus, the optical receiving device presented in Patent Literature 2 performs an operation according to the flow chart shown in FIG. 2 so as to phase-synchronize the received signal with the sampling clock even in the initial state.
Specifically, first, the optical receiving device sets the state of the waveform distortion to a predetermined initial state, loads tap coefficients corresponding to the state of the waveform distortion from a lookup table, and sets them to digital filter section 105.
Thereafter, the optical receiving device determines whether or not the lock status of the clock reproduction is “lock,” namely the waveform distortion of the reproduced signal converges to a predetermined range within a predetermined period of time.
If the lock status is not “lock,” the optical receiving device sets the state of the waveform distortion to the next state, loads tap coefficients corresponding to the resultant state from the lookup table, and sets the tap coefficients to digital filter section 105.
The optical receiving device repeats the above-described operation until the lock status becomes “lock” so as to phase-synchronize the received signal with the sampling clock.
As a result, the received signal can be phase-synchronized with the sampling clock even in the initial state.